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Messages - rsterne

Pages: [1] 2 3 ... 88
1
General Airgun Forum Discussion / Re: TJ .22 Barrel Question
« on: Today at 08:27:17 AM »
I know you can get them in 7/16" and 1/2" OD (maybe even 3/8").... not sure about larger.... Give Mike a call and ask him.... If not, then sleeve it with a Carbon Fiber tube.... or CrMoly if you want the weight.... His barrels are 0.217" groove, 0.213" land, with a 16" twist....

The MM barrels, while made by TJ's, are proprietary, and only available through Jim.... They are tighter and slower, IIRC, designed for JSB pellets only, and are choked....

Bob

2
PCP, C02, and Helium Powered Airguns / Re: Tale of Two BRods
« on: January 21, 2018, 07:35:52 PM »
Today I assembled the regulated tank block I got from Travis back in the summer.... It went well, and after a short learning curve installing shims, I was able to adjust it from 1000 psi up to over 2000.... I didn't try and go higher, but I am sure it will.... It has a 3K burst disc on the output side, and I didn't want to push it.... The adjustment is quite sensitive, you don't have to turn the 10-32 adjusting setscrew very far to move the output by 100 psi.... maybe 1 flat on the allen key or less....

Since the Cothran valve cannot be adjusted using hammer preload (or SSG gap), and needs to have the pressure adjusted to change the velocity, or retune for a different pellet weight.... it was a perfect match for an externally adjustable regulator.... Since there are two excellent pellets made by JSB that are quite different in weight (25.4 gr. and 34.1 gr.) I decided that to install the .25 cal upper on the lower that used the Cothran valve and the adjustable regulated tank block and I installed a 500 cc, 3000 psi bottle on it.... My initial adjustment of 2000 psi drove the 34.1 gr. JSB King Heavies at over 980 fps, but I used the opportunity to find out where the cliff was, and where the SSG gap needed to be adjusted to be close to the cliff, but solidly on the plateau, where the valve is operating stably.... That was with 2 turns of gap.... Then I shot the tank down to about 1700 psi so that I could drop the setpoint adjustment....

When increasing the setpoint pressure, you back out the 10-32 screw, which lowers the HP seat, moving it away from the piston and Belleville stack.... The seat opens, allowing more air to flow to the output side, raising the pressure and pushing the piston towards the (now lower) seat, against the Bellevilles, compressing them until the seat closes at the now higher pressure.... This puts no additional load on the seat, and can be done at any time.... When you want to decrease the output pressure, however, you need to drop the tank pressure below not only your current setpoint, but also below where you want the new setpoint to be.... The reduced pressure on the piston allows it to move away from the seat, and this allows you to turn the 10-32 screw in without putting additional pressure on the seat and possibly damaging it.... I wanted to start at 1800 psi and work my way back up, so I drained the tank down to 1700 and turned the 10-32 adjuster in until I felt slight additional resistance when the seat contacted the piston.... If you do this carefully and slowly, you can feel when they make contact.... When I added more air to the bottle, the gauge on the output side of the tank block moved up slightly, to about 1800 psi.... I shot a few shots, and the velocity was down to about 920 fps, so I simply backed the 10-32 adjusting screw out a fraction at a time, shooting a couple of shots at each setting, until I got my target velocity of 950 fps.... I then shot a couple of shots at each SSG gap setting, and recorded the results, which are on the graph below in red....



You can see the typical "cliff" exhibited by the Cothran valve which starts just before 3 turns out.... The velocity at 3 turns out is very unstable, with an ES of nearly 200 fps, because the valve is about to fall over the cliff.... However, at 2 turns of gap, it is rock solid, with about a 1% ES.... From previous experience I know that is the optimum place to run a Cothran valve, with the lowest hammer strike that allows stable operation and low ES.... I tried the 25.4 gr. pellets, and the velocity was over 1050 fps, so to get them to work I knew I had to lower the setpoint pressure.... I started simply shooting the 25.4 gr. pellets and watched the velocity fall as the pressure dropped.... I was pretty shocked to discover that to get down to 950 fps I had to drop the pressure in the tank all the way down to 1400 psi.... I shot a few more shots, to drop the tank pressure to 1300, and then turned in the 10-32 adjusting screw until I could feel it contact the seat.... I added air to the tank, the output gauge barely budged, and the velocity settled in at about 910 fps at about 1300 psi.... I slowly backed out the pressure adjusting screw, shooting a couple of shot at each adjustment, and stopped when the velocity with the 25.4 gr pellet reached my target of 950 fps.... The gauge indicated 1400 psi.... and I confirmed that by shooting the tank down until the velocity just started to drop, while the tank was still connected to the accurate gauge on my Great White.... Yes, I was getting 51 FPE at only 1400 psi.... I ran through the gap adjustments, and recorded the other curve (in blue) on the graph above.... The optimum gap was at 3 turns out, and the cliff started at 4 turns out, 1 turn more than at 1900 psi.... As expected, the valve requires a bit less hammer strike at 1400 psi than at 1900.... I also shot a few of the 34.1 gr Heavy pellets at that same setting, and that velocity is shown on the graph above as the black dot (872 fps).... So, with the 34.1 gr. pellets, I was getting 57.6 FPE at only 1400 psi.... pretty remarkable, and no adjustment made from the 950 fps setting with the 25.4 gr Kings....

Now that I knew that 3 turns out was the optimum setting for the 25.4 gr. Kings at 1400 psi, I recorded the pressure in the 500 cc bottle (using the gauge on my Great White), and shot 8 shots, on regulator, at 1400 psi.... I averaged the velocity (952 fps = 51.1 FPE), and recorded the pressure drop, which was 200 psi.... That works out to 0.97 FPE/CI, which is pretty low, but we are in new territory here, getting 51 FPE at only 1400 psi.... I repeated the test using the 34.1 gr. Heavies, and as I expected, the pressure drop was exactly the same, at 200 psi.... Since the energy per shot increased to 57.6 FPE, however, the efficiency increased to 1.10 FPE/CI.... I then backed out the 10-32 pressure adjuster to increase the setpoint back to 1900, set the SSG gap at 2 turns, and shot another 8-shot magazine at that setting with the 34.1 gr. Heavies.... The pressure drop was just over 200 psi, so I recorded it as 210 psi, and with the average velocity of 950 fps (68.4 FPE), that works out to an efficiency of 1.24 FPE/CI.... pretty decent at that power level....

The pressure drop per shot for all these tunes runs just about 25-26 psi per shot from the 500 cc bottle.... If we use a fill pressure of 3000 psi, with the 1900 psi setpoint, that should give 42 shots at over 68 FPE with the 34.1 gr Heavies.... and with the 1400 psi setpoint, that should give 64 shots at 51 FPE with the 25.4 gr. Kings and nearly 58 FPE with the Heavies.... I think those are quite acceptable numbers, and tuning back and forth between the 25.4 gr. and 34.1 gr. pellets (both at 950 fps) requires only resetting the external regulator adjustment, and one turn on the SSG gap.... In fact, if you don't care about getting the full 64 shots at the 1400 psi setting, you don't even have to change the SSG gap, you can leave it at 2 turns, and just lose a few shots.... If you have the gun tuned at 1400 psi, you can shoot the Kings at 950 fps and the Heavies at 870 fps, for the same number of shots, with NO adjustment necessary....

Bob


3
I have "known" Bill G (on the internet and through emails) for a couple of years now.... He has a Corbin bullet swage setup, and a knurling setup for airgun bullets, and I helped him with a couple of BBT designs.... It sounds like he is getting more serious about producing a wider range of items for airguns, and I wish him all the best.... I think market research is a good idea, instead of blindly taking off in a direction that HE may believe is the right one.... but carries little interest from the shooters.... Perhaps if you don't see something in his survey that is of particular interest to you (like triggers, as Alan mentioned).... adding your comments in this thread would be appropriate....

Bob

4
PCP, C02, and Helium Powered Airguns / Re: Tale of Two BRods
« on: January 19, 2018, 12:47:19 PM »
Today I tested the ART/SS valve without any jet on the inlet side.... just about a 1/8" hole there.... I still had the preload on the 8 lbs. hammer spring on the SSG set to 0.40" (about 3.2 lbs).... I tested the .30 cal version with the 50 gr. pellets at 1900 psi (only), and realistically there was no significant difference with the jet removed.... The valve still took very little hammer strike to open it.... So, I then swapped out the MDS hammer with the steel core (51 grams) for one with an aluminum core that only weighed 27 grams.... For some unexplained reason I lost about 10 fps from the maximum velocity (which I would not tune for, it would be too inefficient).... but as expected I now needed less SSG gap to get to the cliff.... Here are the results, compared to where I started with this valve (black line)....



Changing to the 0.40" preload from 0.60" is what moved the top of the cliff from 9 turns of gap to 8 turns.... I didn't plot that curve because it essentially tracked the black line except for the earlier cliff.... You can see that with no jet installed (purple line) there was less of a "knee" to the curve.... still some, but there was less velocity adjustment before you got to the cliff.... The same setup but with the 27 gram hammer (the orange line) shifts the cliff to only 5 turns of gap, but I can still reach the plateau with 1 turn of gap.... This gives me an adjustment range of 4 turns, and makes the amount of gap where I would adjust the gun just about where I like to see it.... around 1/8"-3/16".... The only way to reduce it further would be a lighter hammer spring, or reducing the hammer stroke.... I can actually DO that with the aluminum cored hammer, as it has a steel setscrew insert for the striker (currently set flush).... One other thing I noted with the very light hammer.... The range of velocity adjustment between plateau and cliff increases significantly.... leading me to believe that light hammers and this valve would be a great combination....

I did one "efficiency run" by shooting one 7-shot magazine with the velocity set to just over 900 fps with both setups.... Small changes in the exact velocity make big changes in the efficiency at around that velocity with these big ports.... but essentially I believe there is little difference in the efficiency if the velocity is the same with the jet removed.... HOWEVER, it does give you less velocity adjustment just above the cliff, and if you are tuning for efficiency I think you would struggle to get the best combination of power and air usage without the jet.... If all you are interested in is pure power with a light hammer strike, however.... this valve is a BEAST.... It requires MUCH less hammer strike than even the Cothran Powerhouse valve in the testing I have done so far.... I'm impressed....  8) 8) 8)

Bob

5
PCP, C02, and Helium Powered Airguns / Re: Tale of Two BRods
« on: January 18, 2018, 04:25:37 PM »
I reassembled a BRod with the Cothran valve this afternoon just to assess the comparison between the required hammer strike with it and the ART/SS valve.... To my surprise, the new ART/SS valve was easier to open than the Cothran....  :o .... Even with the preload reduced to only 0.40" (3 lbs.), the Cothran valve fell over the cliff at a 3 turn smaller gap than the ART/SS valve, with the same pellet at the same pressure.... Now BOTH these valves are a LOT easier to open than a conventional PCP valve of the same dimensions.... That is the big advantage to balanced valves, you can use huge ports, throat and poppet, and not have to end up with a gun that is impossible to cock....

I rechecked the Cothran valve, and it remains stubbornly "ON" or "OFF".... You really have no control over the velocity with the hammer strike, you need to change the setpoint pressure to do that.... Yes, the Cothran valve uses less air when tuned just above the cliff, but it cant be turned down a bit before it ceases to cycle properly and the velocity drops to near nothing.... It is when operating just above the cliff that the ART/SS valve can be tuned for good efficiency while remaining stable (unless you get too close to the cliff, as mentioned earlier).... That region with the Cothran valve is very narrow, and when you get close to the cliff the velocity becomes unstable, although if you get it just right, it also exhibits improved efficiency.... It is just way fussier to adjust than the ART/SS valve....

To make large adjustments to the velocity with the ART/SS valve (at least in my large ported version) you still need to alter the setpoint pressure.... However, having a 100-200 fps range of adjustment without changing the pressure sure makes it easier to find a suitable tune.... It seems that the heavier the bullet, the wider the velocity adjustment range.... I suspect that with a stock ported ART/SS valve the adjustment range might be even wider.... It looks like I may have to find a weaker spring to reduce the SSG gap down to the desired level when running regulated at 1900 psi.... but before I make that change, I think I will completely remove the front jet and see what happens....

Bob

6
PCP, C02, and Helium Powered Airguns / Re: Tale of Two BRods
« on: January 18, 2018, 11:46:38 AM »
Here is the data for the .25 cal BRod using the ART/SS valve.... The methodology was the same as for the .30 cal.... except the pellets used were the 25.4 gr. Kings and 34.1 gr. King Heavies at 1900 psi and a 52.7 gr. RN .25 ACP bullet at 3000 psi.... Here are the results....



The SSG gap where the gun fell off the cliff was virtually the same at the same pressure, about 7 turns at 3000 psi and 9 turns at 1900 psi.... I was using the same hammer and spring, all that changed was the upper (receiver and barrel).... This gun drives the Kings way too fast, nearly Supersonic at 1900 psi, but the King Heavies look to be a great choice at this pressure.... At 7 turns of gap (just over 3/8") the velocity is 966 fps (70.7 FPE) but the efficiency is a bit low, at only 1.01 FPE/CI.... Still, this is pretty impressive performance at only 1900 psi, and should yield about 32 shots at 70 FPE from the 500 cc bottle.... nothing to sneeze at....

As a bullet shooter at 3000 psi, the gun is solidly over 100 FPE, peaking at 115 FPE.... This is the highest FPE (by a whisker) I have seen with the .25 cal BRod at this bullet weight, and with a 60 gr. bullet would be even higher I am sure.... I didn't do any efficiency testing at that pressure, as I am short of cast bullets at the moment, and don't want to stop testing to make more.... Once again, if you crank in actual preload on the SSG, the gun roars and kicks as it blows air out the muzzle after the bullet is long gone.... It appears obvious that this valve (at least with the big ports) needs either an SSG or SSS to have any chance of behaving in a civilized manner.... It does offer some range of velocity adjustment, and the efficiency, like the Cothran valve, is best when operating just above the cliff.... However, with the ART/SS valve, that range is a lot broader and easier to tune to....

With the ART/SS valve using a 1/4" exhaust port, my BRods will be great with pellets when regulated at 1900 psi.... and are capable of shooting with bullets in both calibers at 3000 psi.... I'm pretty please with the results for initial testing.... I do need to try less preload on the spring to try and reduce the gap on the SSG, particularly when regulated.... I hope to get to that soon....

Bob

7
PCP, C02, and Helium Powered Airguns / Re: Tale of Two BRods
« on: January 17, 2018, 03:45:17 PM »
Here is the first set of results for my BRod .30 cal with the ART/SS valve installed.... I tethered it at 3000 psi (a more accurate, digital gauge says my regulator was not set for 2900 as I previously thought), and tried the 50 gr. JSB/Daystates and also the 67 gr. NOE BBTs.... I tried all the SSG settings from maximum preload (way too much), through zero gap, to 10 turns of gap (over 1/2").... I then repeated the procedure with the 45 gr. JSBs and 50 gr. JSB/Daystates tethered at 1900 psi.... Lastly, I connected up my 500 cc bottle which has a regulated 1900 psi output and shot one 7-shot magazine of the 50 gr. pellets at several gap settings to find the pressure drop and calculate the efficiency.... Here are the results....



First some comments about this valve.... IT OPENS EASILY.... nowhere near the hammer strike required of my modded MRod valve.... I will be doing the same testing procedure with my Cothran valve to get an idea of the relative hammer strike required, but don't have that data yet.... It is much more responsive to changes in hammer strike than the Cothran, although still exhibits a VERY sharp velocity drop if you try and tune it for tiny sips of air (ie low velocity).... This sudden drop could well be due to the jet size installed in the inlet of the valve, I only got one with the valve, presumably a "stock" one, whatever that is.... and I can tell you it's pretty tiny.... With that tiny jet, and the huge drilled out exhaust port, I may be operating the valve outside its normal parameters.... in fact you can almost bet on it.... I hope Travis has some ideas to try and will send me some other jets to play with.... not that it really matters, I never tune my guns on the downslope anyways.... If I wanted a lower velocity, I would reduce the regulator setpoint and move back up onto the knee of the curve....

I am using an MDS hammer with a 3/4" steel core that weighs 51 grams.... The hammer spring is 0.48" OD x 3.00" long, and is made of only 0.051" wire, so has a spring rate of just 8 lb/in.... It is currently set up on the SSG with 0.60" of preload (4.8 lbs.).... and when set to zero gap, with the hammer stroke I have of 0.79" takes a maximum force of only 11 lbs. to cock the gun.... If you notice from the data in the graph, I can run the SSG gap at about 6 turns out (0.33") at 1900 psi (cocking force 8.5 lbs) and about 3 turns out (0.165") at 3000 psi (cocking force under 10 lbs) with hardly any velocity loss.... Those settings are basically at the top of the "knee".... With the 50 gr. pellet at 1900 psi, at 6 turns out, the velocity is 922 fps (94 FPE) and the efficiency is 1.04 FPE/CI.... The knee is quite long and gentle, making velocity adjustment quite insensitive, until you approach the "cliff".... Just before you get there, the velocity seems to get unstable, and the ES increases.... and then suddenly you are teetering on the edge of the cliff, where just a quarter turn more gap and the velocity tanks to under 400 fps....

The maximum performance (plateau velocity) with this valve is a few fps better than my modded MRod valve or the Cothran valve at 1900 psi.... and virtually the same with the 67 gr. BBT at 3000 psi.... but with the 50 gr. pellet at 3000 psi it was about 30 fps faster (1100 fps).... However, it also had virtually no velocity drop as you increased the SSG gap until it suddenly fell off the cliff.... almost acting like a Cothran valve.... However, this is so far out a normal condition (50 gr. pellet at 1100 fps) it doesn't really matter.... it's more an anecdote.... One other thing of note.... If I reduced the SSG gap to negative (ie introduced preload) the air consumption, report, and recoil all went "stupid".... I can only assume that the closing forces on the balanced poppet are so small that they are overwhelmed by the 5 lbs. preload on the SSG, and it increases dwell to the point the valve is still wide open when the pellet leaves the muzzle.... and YES, it's THAT dramatic.... When I was testing the pressure drop with 2 turns of preload, I actually got 640 psi of drop in my 500 cc bottle in just 3 shots.... compared to 370 psi drop in 7 shots with zero SSG gap.... Pretty much BOOM when you pull the trigger, complete with 6" or more of muzzle jump....  :o :o

The 8 lb/in. spring is OK for use at 3000 psi, but realistically I need to either reduce the preload on the SSG, or fit an even weaker spring for use at 1900 psi.... The SSG gap is too large at that pressure, and I might find a velocity difference shooting up or downhill from gravity slowing the hammer.... This change would make the gun even easier to cock, which seems silly, as it's sooooooooo easy now.... I can tell you, I'm SURE impressed at the reduction in cocking force with the ART/SS valve.... Travis and Lloyd sure came up with a winner here.... I can see that I will end up converting all my Big Bores and bullet shooters over, to get rid of the massive, long stroke, hard to cock hammers.... Incidently, I resized a 109 gr. Lee HP in .308 cal down to 0.300", and although it was still hard to chamber, I got 811 fps with 2 turns of preload (159 FPE) at 3000 psi.... accompanied by a huge roar and muzzle jump.... Yes, this valve can move a LOT of air....

Bob

8
PCP, C02, and Helium Powered Airguns / Re: Tale of Two BRods
« on: January 16, 2018, 09:14:11 PM »
This project has been on the shelf over the summer, both for lack of shop time and because I have been waiting to received one of the new ART/SS valves to try.... The hope is that it will match the Cothran, both in performance and ease of opening.... with the added benefit of being able to adjust the velocity with hammer strike, something you can't do with the Powerhouse valve.... you have to change the pressure to change the velocity.... Anyways, the wait is over and my valve arrived today, many thanks, Travis....  8)

The valve is a stock JSA "SS Balanced Valve" as they supply for the MRods.... It comes with a 3/16" exhaust port and the stock recess for the transfer port, which would not work in my BRods, as they are set up for a 3/8" OD transfer port with a 1/4" exhaust port.... Travis assured me that there was plenty of meat to enlarge the ports, and there certainly was.... I usually angle my exhaust ports about 20 deg. but with that big a port and no experience with this valve, I choose 15 deg. instead, and I'm glad I did.... At 20 deg. with a 1/4" port, I think I would have broken into the back of the valve and ruined the seat.... The valve throat is 0.281", which works out to the same area, once you subtract the 1/8" stem, as the 1/4" exhaust ports.... This is the same size throat as what I had on my modded MRod valve with the PEEK poppet, but the area is smaller than the Cothran valve.... I didn't want to drill out the throat at this time, but I did blend the new 1/4" exhaust port into the throat with a spherical Dremel burr.... There is certainly lots of port area for lots of flow....

I got the valve installed in the tube for one of my BRods and it seems to be holding pressure.... I will leave it overnight and plan on doing extensive testing tomorrow, tethering the gun at 1900 and 2900 psi in both .25 and .30 cal to get the baseline preload curves and assess what the opening forces are like compared to the Cothran valve and my modded MRod valve.... This should be FUN !!!

Bob

9
Airgun Ammo / Re: Inventory Run - 63 gr. FN in .257 cal at NOE - In Store
« on: January 16, 2018, 06:08:38 PM »
This run is complete, and the moulds are in the NOE Store.... http://noebulletmolds.com/NV/index.php?cPath=26_180&osCsid=dbpe0lr3s55j4n956fgcanpro2

If you subscribed, you should have already received your discount code via PM or email....

Bob

10
Is it possible that your CO2 source tank HAS a dip tube and that you shouldn't be inverting it?.... just a thought....

Bob

11
Are you inverting the source CO2 tank/.... or does it have a dip tube?.... It sounds like you are not getting a proper fill of liquid CO2 in your 88 gr. cylinder.... A partial CO2 fill of it and then vent it to chill it may be necessary to get the proper amount of CO2 liquid in it.... Of course weighing it before and after is the only way to know for sure....

Bob

12
Airgun Ammo / Re: Jkhan noblesse .22 and 30g bbt
« on: January 15, 2018, 12:38:51 PM »
Quote
How did you size the nose diff than the bands?

I don't think he did.... ? .... I designed the molds that way so that guys could try a bullet where the nose fit the lands and the rear band fit the chamber / grooves.... I don't think it was necessary, just gave another option.... that goes away when you size them....

Bob

13
A perfect example would be the new Umarex Gauntlet.... There is enough air onboard to get 40 or more shots at 35 FPE (950ish with 18.1 gr.) and not exceed 1500 psi regulated, so that they could still use a 1.8K burst disc.... I know from personal experience that JSB Exact Heavies at that velocity are a devastating 100 yard GS round, and good for 'Chucks at half that range with a brain shot.... I often get through a day's Varmint hunting without having to refill my 13 CI tank on my 2260 HPA, and I've never had enough targets to fill more than once mid-day.... The manufacturers don't seem interested in tuning for the right combination of heavy pellet and the FPE that it can carry.... they seem more interested in crowing about velocity (with a light pellet) or shot count (at low FPE).... than in building a great hunting gun.... This gun, tuned the way it is, is peaking at 1600 psi at 35 FPE with the 18 gr., and nearly dead flat (just over a 1% ES) from 1500 - 1700.... and over 40 FPE with the 25 gr. pellets.... and there is nothing really special or complicated about it.... all could be done by the manufacturer....

Bob

14
As promised, here are the results.... First, the data showing velocity vs preload (or gap) on the hammer spring, at 1800 psi, with the two pellet tested, the 18.1 gr. and 25.4 gr. JSB Exacts....



I was delighted that the new PEEK poppet with the small head and small stem was so easy to open.... Of course the 50% increase in hammer stroke would have helped a lot to make up the difference between CO2 pressures and the 1800 psi I am running on this HPA conversion.... Still, I am only using a stock QB hammer spring, and even with 1/16" of gap (2 turns) between the spring and hammer when uncocked the gun is able to reach maximum velocity with this combination of porting and pressure.... The downslope of the velocity curves are very steep (it doesn't take much of an increase in gap to drop when below 800 fps).... and the knee of the tuning curve is very pronounced.... making adjustment of the gap quite sensitive.... Just 1/2 turn can move you from a perfect tune to starting at the peak, or starting to get valve lock at the beginning of the string, with a big velocity drop.... I played a bit with the adjustments and came up with the following strings, starting from an 1800 psi fill....



The performance is very similar to the Beeman Chief I just built, except for a lot more shots from the larger reservoir (290 cc instead of 135 cc).... The velocities are just slightly lower than the Chief, a result of the slightly lower fill pressure (1800 vs. 2000).... I don't think the larger porting on the Chief actually made any significant difference, and I am wondering if the 3/32" bolt probe, which was shared by both guns (which are both .22 cal), is the limiting factor to the flow.... In theory the area around the probe in the barrel (chamber) is the equivalent of a hole 0.198" in diameter, and on the Chief all the ports are larger than that.... while in this QB conversion the transfer and barrel ports are only 0.172".... It may be that just as we like to see the throat area (after the valve stem is deducted) a bit larger than the porting.... the same thing applies to the chamber and the bolt probe.... I know that using a retracting bolt to completely clear any obstruction from the air path in the barrel helps performance in bullet shooters, where we are after full bore area porting.... It may well be that once you go larger than 80% of bore for the transfer port (which is as large as you can go with a drilled barrel port without possible loading problems).... that to see full benefit you have to retract the bolt out of the airflow.... With a strictly pellet shooter, I could probably get away with a smaller probe (1/16") and still seat the pellet fine.... but with a bullet shooter (which I may do with the Chief) such a small probe would sink into the back of the bullet, because you have to push harder to chamber them.... In any case, a smaller probe would not likely change anything on this build, with the porting less than 80% of the cailber....

The astonishing thing, for me, is the efficiency of this gun.... The total reservoir volume (88 gr. cylinders at 121 cc each plus the QB78 air tube) is about 290 cc or a whisker over.... let's call it 18 CI (295 cc).... From an 1800 psi fill, with strings with a 4% ES, the strings end at 1370-1380 psi, using only about 420-430 psi for the string.... For the 23 shots at an average of 934 fps with the 18.1 gr. JSBs (35.0 FPE) that works out to 1.51 FPE/CI.... and for the 20 shot string with the 25.4 gr. JSBs at an average of 846 fps (40.3 FPE) it works out to 1.55 FPE/CI.... The pressure only drops about 20 psi per shot.... When I reduced the spring gap a turn with the 25.4 gr. pellets, and got that short, declining string (in blue), the efficiency dropped to 1.16 FPE/CI, for 10 shots within a 4% ES averaging 874 fps (43.1 FPE) on 300 psi of air (30 psi per shot).... It just shows how quickly the air usage goes up if you try and get too much power, using 50% more air to get only 7% more FPE....

I am extremely pleased with how this HPA conversion turned out.... The QB has proven once again how versatile it is.... and this one has enough air on board to be quite a powerhouse, despite its modest 1800 psi fill pressure....

Bob

15
I finished up the receiver and barrel mods today and made a new hammer.... Here is a photo of the hammers, old and new....



The cocking pin is in the same place relative to the front of the hammer, but the sear notch is forward 1/4".... and the back of the hammer is shortened to save some weight.... The new hammer weighs 43 grams instead of 55 grams.... a reduction of 22%.... The hammer stroke is now 3/4" instead of 1/2".... and using the "cock-on-open" parts shown above and a stock QB hammer spring I have 10 turns of preload adjustment, from 3 turns of preload to 7 turns of gap, using the 10-32 adjusting screw.... It ends up that was perfect....

The barrel port was drilled out to 11/64" (0.172") or 78% of the bore.... and a transfer port made from 1/4" Teflon rod with the same size hole.... I drilled a hole in the loading tray area of the barrel to allow it to be assembled to the receiver and the entire assembly fitted to the gun.... since the flat head screw under the loading tray can be accessed with a Phillips screwdriver through that hole.... That flat head screw now lives (permanently) trapped between the barrel and receiver.... I also installed two 6-32 setscrews in the sides of the receiver that tighten against the back sides of the barrel loading tray.... which really stiffens up the assembly.... The bolt was shortened (flow through portion cut off) and a 3/32" probe fitted that pushes the pellets just ahead of the barrel port.... I shortened the stock to just clear the back of the 88 gr. cylinders.... and assembled the gun for the first time....



I weighs exactly 8 lbs. as shown.... I have done some shooting, and the performance exceeds what I was hoping for.... A report will follow once I have a chance to organize the data.... but the maximum velocities are 1000 fps with 18.1 gr. (40.2 FPE) pellets and 910 fps with 25.4 gr (46.7 FPE).... with both pellets up on the plateau at 1800 psi....  8) .... The knee of the curve occurs with some gap between the hammer spring and hammer, so I expect to see the benefits of a Short Stiff Spring installation, even though I am using the stock spring (with a gap)....

Bob


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